Wide bandgap semiconductors such as nitride semiconductors containing silicon carbide (SiC) and gallium (Ga) are expected as materials for next-generation semiconductor devices. The wide bandgap semiconductor has a wider bandgap so that breakdown field strength increases as compared to silicon (Si). It is possible to realize a transistor with a high breakdown voltage by utilizing such a characteristic.
The high breakdown voltage transistor requires to have a high threshold voltage in order to realize a stable circuit operation. However, in a transistor using silicon carbide, for example, there occurs a problem that carrier mobility is lowered if an impurity concentration in a region where a channel is formed is increased in order to increase the threshold voltage. In addition, for example, in a high electron mobility transistor (HEMT) using a nitride semiconductor containing gallium (Ga), a two-dimensional electron gas is present right below a gate electrode, and thus, it is difficult to increase the threshold voltage.